# One-to-many relations (Prisma ORM v7) (/docs/orm/v7/prisma-schema/data-model/relations/one-to-many-relations)

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How to define and work with one-to-many relations in Prisma.

Location: ORM > v7 > Prisma Schema > Data Model > Relations > One-to-many relations

One-to-many (1-n) relations connect one record on one side to zero or more records on the other side:

```prisma
model User {
  id    Int    @id @default(autoincrement())
  posts Post[]
}

model Post {
  id       Int  @id @default(autoincrement())
  author   User @relation(fields: [authorId], references: [id])
  authorId Int
}
```

This expresses:

* A user can have zero or more posts
* A post must always have an author

You can also reference a field other than the primary key. The referenced field on the parent model must be unique, so mark it with `@unique` (an `@id` field is already unique). To reference several fields at once, they must together form a `@@id` or `@@unique`; see [Multi-field relations](#multi-field-relations-relational-databases-only).

```prisma
model User {
  id    Int    @id @default(autoincrement())
  email String @unique
  posts Post[]
}

model Post {
  id          Int    @id @default(autoincrement())
  authorEmail String
  author      User   @relation(fields: [authorEmail], references: [email])
}
```

## Multi-field relations (relational databases only) [#multi-field-relations-relational-databases-only]

```prisma
model User {
  firstName String
  lastName  String
  post      Post[]
  @@id([firstName, lastName])
}

model Post {
  id              Int    @id @default(autoincrement())
  author          User   @relation(fields: [authorFirstName, authorLastName], references: [firstName, lastName])
  authorFirstName String
  authorLastName  String
}
```

## 1-n in the database [#1-n-in-the-database]

The difference between 1-1 and 1-n is that in a 1-1 relation the foreign key must have a `UNIQUE` constraint. Without `UNIQUE`, multiple records can point to the same parent, making it 1-n.

## Required vs optional relation fields [#required-vs-optional-relation-fields]

The annotated relation field and relation scalar can be either optional or mandatory. The list side is always mandatory.

**Optional 1-n** (can create Post without User):

```prisma
model User {
  id    Int    @id @default(autoincrement())
  posts Post[]
}

model Post {
  id       Int   @id @default(autoincrement())
  author   User? @relation(fields: [authorId], references: [id])
  authorId Int?
}
```

**Mandatory 1-n** (must assign User when creating Post):

```prisma
model User {
  id    Int    @id @default(autoincrement())
  posts Post[]
}

model Post {
  id       Int  @id @default(autoincrement())
  author   User @relation(fields: [authorId], references: [id])
  authorId Int
}
```

## Related pages

- [`Many-to-many relations`](https://www.prisma.io/docs/orm/v7/prisma-schema/data-model/relations/many-to-many-relations): How to define and work with many-to-many relations in Prisma.
- [`One-to-one relations`](https://www.prisma.io/docs/orm/v7/prisma-schema/data-model/relations/one-to-one-relations): How to define and work with one-to-one relations in Prisma.
- [`Referential actions`](https://www.prisma.io/docs/orm/v7/prisma-schema/data-model/relations/referential-actions): Referential actions let you define the update and delete behavior of related models on the database level
- [`Relation mode`](https://www.prisma.io/docs/orm/v7/prisma-schema/data-model/relations/relation-mode): Manage relations between records with relation modes in Prisma
- [`Self-relations`](https://www.prisma.io/docs/orm/v7/prisma-schema/data-model/relations/self-relations): How to define and work with self-relations in Prisma.